Widely known are two-stroke engines in which scavenging air is introduced to the combustion chamber during the downward stroke of the piston, and an air/fuel mixture is introduced to the combustion chamber after exhausting combustion gas. These methods of introduction include feeding scavenging air to a scavenging channel connected to the crankcase and the fuel chamber and also feeding an air/fuel mixture to the crankcase. Alternatively, scavenger air is introduced to the fuel chamber and an air/fuel mixture is introduced to the crankcase to introduce scavenging air to the fuel chamber in the order described above.
The flow rate of scavenging air and the air/fuel mixture must be made proportional in order to prevent incomplete combustion, stabilize engine operation, and so forth. An air valve is therefore provided to the air channel for supplying scavenging air to the engine. The air valve is operated in coordination with the throttle valve of the carburetor. In this case, when the valves are butterfly throttle valves, the air valve is also a butterfly valve, and it is common to link these with a widely-known link mechanism, as cited in Japanese Patent Application Laid-open Nos. 9-125966 and 2000-314350 and International Patent Application WO01/51782A1, for example.
When the carburetor is provided with a choke valve, start-up is adequately performed by causing high negative pressure to act on the main nozzle in a condition in which the choke valve is closed and the throttle valve is opened slightly at low temperatures. At this time, the air valve must be closed. When a link mechanism (refer to Japanese Patent Application Laid-open No. 2000-314350) configured so that the air valve is opened after the throttle valve has been opened to a certain degree is used in order to achieve this goal or to prevent the air/fuel mixture from becoming thin during low loads, a drawback tends to arise in that the operator cannot determine the point in time at which the air valve begins to open, so the throttle valve is allowed to open until the air valve opens, making it impossible to obtain a high density air/fuel mixture required for start-up at low temperatures.
As a countermeasure, it is possible to consider interlocking a throttle valve and a choke valve with a link mechanism, and opening the throttle valve in a range in which the air valve does not open when the choke valve is closed. However, there is a drawback when this countermeasure is adopted in that two mutually independent link mechanisms are used, the configuration becomes complex, the number of components greatly increases, and a fairly large installation space must be set aside when mounting the structure on an engine.